How can natural and humans factors affect trends in nutrient concentrations in streams?
Throughout the United States, nutrient sources change at different rates and in different directions (increase or decrease), making it difficult to determine their effect on nutrient trends in streams. Further, data on many nutrient sources (for example, changes in fertilizer applications, concentrated animal feeding operations) and management practices are of unknown reliability, are inconsistent over time and space, or are not collected at all. The period of NAWQA assessment of changes in groundwater (1988-2004) and streams (1992-2003) was a period with relatively stable nutrient inputs from nonpoint sources that followed three decades (1950 through 1980) during which nitrogen and phosphorus fertilizer use increased 10-fold and 4-fold respectively. During the NAWQA study period, populations have increased and many measures have also been implemented to reduce input of nutrients to the environment from both urban and agricultural sources. Human activities are not the only influence on
